Added a ShowUIErrors setting on WebAuthenticator

This allows the caller to opt out of any UI being shown so that they can handle the error as they see fit.

Note:
- I did not update the FormAuthenticatorController as I do not use it and I wasn't sure how to test it. 
- I could not find any relevant UI tests, so I have verified this new setting works as desired by consuming it from the Mobile Services SDK
Style: Space before parentheses ` (`.